1. Field of the Invention
The present invention relates to a multiplexer for multiplexing or demultiplexing signals in three different frequency bands, and more particularly, to a 1-to-3 multiplexer/demultiplexer integrated in one component.
2. Description of the Related Art
In the field of mobile communication devices, such as a portable telephone, three signals in different frequency bands, such as an AMPS signal, a GPS signal, and a PCS signal, are used by a single communication device.
When signals in three frequency bands are used by a single communication device, if a multiplexer/demultiplexer is used to multiplex or demultiplex three signals in different frequency bands, a circuit at a stage preceding or following the multiplexer/demultiplexer can be used in common for all three frequency bands. This allows for reductions in the size and cost of the communication device.
It is known in the art to use a diplexer to combine or separate two signals in different frequency bands. It is also known in the art to use three signals in different frequency bands by using a diplexer in conjunction with cascaded switches as is taught, for example, by Japanese Unexamined Patent Application Publication No. 2000-165288. An example configuration for the above-described purpose is illustrated in the form of a block diagram in FIG. 7. In the example illustrated in FIG. 7, the above technique is used in the front end of a triple band portable telephone capable of using DCS, PCS, and GSM signals. In transmission operation, as illustrated in FIG. 7, a diplexer 2 connected with an antenna 1 combines transmission signals output from the DCS, PCS, and/or GSM units. During a receiving operation, the diplexer 2 separates a received signal into the DCS, PCS, and/or GSM units. A first RF switch 3 switches the connection of the antenna between transmitters of DCS and PCS units and receivers of DCS and PCS units. A second RF switch 4 switches the connection of the receivers between a receiver Rxd of the DCS unit and a receiver Rxp of the PCS unit. A third RF switch 5 switches the connection of the antenna between a receiver Txg of the GSM unit and a receiver Rxg of the GSM unit. A first filter 6 passes transmission/reception signals of DCS and PCS units. A second filter 7 passes transmission/reception signals of the GSM unit.
In the 1-to-3 multiplexer/demultiplexer illustrated in FIG. 7, transmission/reception signals must pass through the diplexer 2 and two RF switches 3 and 4. This results in an increase in insertion loss compared with the case in which signals pass through only one RF switch in addition to the diplexer 2. Besides, diodes used in RF switches are expensive, and thus the increase in the number of RF switches results in an increase in total cost.
A 1-to-3 multiplexer/demultiplexer can also be realized without using an RF switch, for example, as illustrated in FIG. 8A or 8B. In the example illustrated in FIG. 8A, a first multiplexer/demultiplexer 11 includes a lowpass filter 13 and a highpass filter 14, wherein the lowpass filter 13 passes a signal in a first frequency band and attenuates a signal in second and third frequency bands, and the highpass filter 14 passes a signal in second and third frequency bands and attenuates a signal in the first frequency band. A second multiplexer/demultiplexer 12 includes a lowpass filter 15 and a highpass filter 16, wherein the lowpass filter 15 passes a signal in the second frequency band and attenuates a signal in the third frequency band, and the highpass filter 16 passes a signal in the third frequency band and attenuates a signal in the second frequency band. The first multiplexer/demultiplexer 11 is connected with a common input/output port 17 connected with an antenna. The first multiplexer/demultiplexer 11 is also connected with the second multiplexer/demultiplexer 12 and an input/output port 18 connected with a first-frequency-band signal processing section. The second multiplexer/demultiplexer 12 is also connected with the first multiplexer/demultiplexer 11, an input/output port 19 connected with a second-frequency-band signal processing section, and an input/output port 20 connected with a third-frequency-band signal processing section.
In the example illustrated in FIG. 8B, a first multiplexer/demultiplexer 11 includes a lowpass filter 13 and a highpass filter 14, wherein the lowpass filter 13 passes signals in a first frequency band and a second frequency band and attenuates a signal in third frequency bands, and the highpass filter 14 passes a signal in the third frequency band and attenuates signals in the first and second frequency bands. A second multiplexer/demultiplexer 12 includes a lowpass filter 15 and a highpass filter 16, wherein the lowpass filter 15 passes a signal in the first frequency band and attenuates a signal in the second frequency band, and the highpass filter 16 passes a signal in the second frequency band and attenuates a signal in the first frequency band. The first multiplexer/demultiplexer 11 is connected with a common input/output port 17 connected with an antenna. The first multiplexer/demultiplexer 11 is also connected with the second multiplexer/demultiplexer 12 and an input/output port 20 connected with a third-frequency-band signal processing section. The second multiplexer/demultiplexer 12 is connected with the first multiplexer/demultiplexer 11, an input/output port 18, and an input/output port 19, wherein the input/output port 18 is also connected with a first frequency band signal processing section, and the input/output port 19 is also connected with the third-frequency-band signal processing section.
In both examples illustrated in FIGS. 8A and 8B, each 1-to-3 multiplexer/demultiplexer includes two 1-to-2 multiplexer/demultiplexers that are connected with each other. As a result, the signal in the second frequency band has to pass through two filters, and thus the signal encounters a large insertion loss. Further, the use of two multiplexer/demultiplexers increases the number of components and the size of the component.